In measurement operation, the fill level measuring device sends microwaves by means of an antenna and receives reflection signals reflected back by the surface of the fill substance after a travel time dependent on the fill level to be measured, and determines the fill level based on the travel time. Such contactless measuring arrangements are applied in a great number of sectors of industry, e.g. in the processing, chemical and food industries. In such case, the fill level measuring device is mounted on the container above the fill substance and its antenna oriented toward the fill substance.
All known methods, which enable relatively short distances to be measured by means of reflected microwaves, can be applied to determine the travel times. The best known examples are pulse radar and frequency modulated, continuous wave radar (FMCW radar).
In the case of pulse radar, short microwave transmission pulses are periodically sent, which reflect off the surface of the fill substance and are received back after a travel time dependent on distance. An echo function is derived based on the received signal; the echo function shows received signal amplitude as a function of time. Each value of this echo function corresponds to the amplitude of an echo reflected at a determined distance from the antenna.
In the FMCW method, a microwave signal, which is periodically frequency modulated linearly, for example, according to a saw tooth function, is sent continuously. Consequently, the frequency of the echo signal received has a frequency difference compared to the instantaneous frequency that the transmission signal has at the point in time of the reception; the frequency difference depends on the travel time of the microwave signal and its echo signal. The frequency difference between transmission signal and the received signal, which can be gained by mixing both signals and evaluating the Fourier spectrum of the mixed signal, thus corresponds to the distance of the reflecting area from the antenna. Additionally, the amplitudes of the spectral lines of the frequency spectrum gained through a Fourier transformation correspond to the echo amplitudes. Consequently, this Fourier spectrum represents the echo function in this case.
From the echo function, at least one wanted echo is determined, which corresponds to the reflection of the transmission signal off the surface of the fill substance. With a known propagation velocity of the microwaves, the distance, through which the microwaves travel on their way from the measuring device to the surface of the fill substance and back, is directly determinable from the travel time of the wanted echo. On the basis of the installed height of the fill level measuring device over the container, the fill level sought can be directly calculated.
There are a large number of applications, in which it is required or at least desirable, to transmit the microwave signals through a gas tight feedthrough into the container and to receive its reflection signals through such feedthrough. This is always the case e.g. when a gas tight separation is required for the process; the container is sealed gas tightly. Furthermore, such a feedthrough, which prevents gas diffusion is required e.g. when an encapsulation of the measuring device electronics is prescribed, e.g., for explosion protection reasons. This requirement is especially relevant for fill level measuring devices using high frequency microwave signals, especially microwave signals with frequencies of 70 GHz or higher, for fill level measurement, since very high power levels are converted in these measuring devices in the measuring device electronics.
Gas tight feedthroughs can be realized, e.g., in the form of hollow conductor feedthroughs. In such case, a window comprising a microwave permeable insulator is inserted into a hollow conductor. Conventional methods for this are soldering in ceramic windows or glazing in glass windows. Glazings involve, most often, compression type glass feedthroughs or as so called fitted feedthroughs. In such case, the hollow conductor, a metal hollow conductor as a rule, is shrink fit onto a glass window with similar coefficient of thermal expansion. In this way, the sealing of the feedthrough is assured.
DE 41 00 922 A1 describes a hollow conductor feedthrough, which can be applied in applications, in which the feedthrough is exposed to high pressure and/or temperature fluctuations. The feedthrough comprises a hollow conductor in which two windows comprising a material permeable by electromagnetic waves are arranged as mirror images relative to one another; the hollow conductor is divided into two separate segments by the windows. Each window has a cylindrical section and a conical section adjoining thereto. The conical sections are each embedded in a correspondingly conically formed socket in the hollow conductor in order to achieve a high resistance of the feedthrough against pressure and/or temperature fluctuations. As evident from the figures of this application, a very thick solid window arises thereby. Arranged on the free ends of the cylindrical sections are λ/4 transformers.
However, the windows of such feedthroughs lead, as a rule, to a strong degradation of the quality of the measuring signals. In relation to this it has been shown that these disadvantageous effects of the feedthrough become more serious as the thickness of the window increases in comparison to the wavelength of the microwave signals. Reflections and multiple reflections, which occur at the transitions to and from the window and in the window, are the main cause for these disadvantageous effects. In this way disturbance signals arise, which are superimposed on the actual measurement signal and therewith lead to a degradation of the quality of the measuring signals.